This invention relates to a pattern forming method and, in particular, relates to a pattern forming method that processes a coating layer over a substrate into a predetermined pattern by partly removing the coating layer and then forms a recess in an underlying layer below the coating layer at its portion corresponding to at least a portion of a region where the coating layer is removed.
This invention also relates to a method of manufacturing a phase shift mask for use in transferring a fine pattern of an LSI or the like by the use of a projection exposure apparatus.
Following higher integration and circuit pattern miniaturization in large-scale integrated circuits (LSIs), phase shift masks have been proposed and put to practical use as a super-resolution technique in the photolithography.
There have been proposed various types of phase shift masks, such as Levenson type, edge emphasizing type, auxiliary pattern type, chromeless type, and halftone type. For example, the Levenson type phase shift mask has a light-shielding pattern formed by a metal film such as a chromium film, or the like on a transparent substrate. The Levenson type phase shift mask is configured such that, in the case where light-shielding portions and light-transmitting portions are alternately arranged like a line-and-space pattern, the phases of transmitted lights through the light-transmitting portions adjacent to each other via each light-shielding portion are shifted by 180 degrees. Because of the shift in phase between the transmitted lights through the light-transmitting portions, a reduction in resolution due to interference between diffracted lights can be prevented to thereby achieve an improvement in resolution of the line-and-space pattern.
In such a phase shift mask, an optical path length difference of [λ(2m−1)/2] (m is a natural number) is generated between transmitted lights, each having a wavelength λ, through the light-transmitting portions adjacent to each other via the light-shielding portion, thereby causing the phase difference of 180 degrees between the transmitted lights. In order to generate such an optical path length difference, a difference d between the thicknesses of the transparent substrate at the light-transmitting portions adjacent to each other via the light-shielding portion should satisfy [d=λ(2m−1)/2n] where n represents a refractive index of the transparent substrate.
In order to generate the difference between the thicknesses of the transparent substrate at the adjacent light-transmitting portions in the phase shift mask, a transparent thin film is coated on the transparent substrate at one of the light-transmitting portions to thereby increase the thickness or the transparent substrate is etched at one of the light-transmitting portions to thereby reduce the thickness. That is, in the shifter coated type (convex type) phase shift mask, the transparent substrate is covered with the transparent thin film (shifter) having the thickness d (=λ(2m−1)/2n) at the phase shift portion.
On the other hand, in the etching type phase shift mask in which the transparent substrate is etched, the transparent substrate is etched by the depth d (=λ(2m−1)/2n) at the phase shift portion. The light-transmitting portion not coated with the transparent thin film or etched serves as a non-phase-shift portion. Note that in the case where the adjacent light-transmitting portions have a shallow etched portion and a deep etched portion, respectively, the shallow etched portion serves as a non-phase-shift portion.
Further, as a phase shift mask for forming an isolated pattern such as contact holes, the auxiliary pattern type phase shift mask has been proposed as described in Japanese Patent (JP-B) No. 2710967 (Patent Document 1).
FIGS. 1A to 1C show the structures of auxiliary pattern type phase shift masks, wherein FIG. 1A is a plan view of the auxiliary pattern type phase shift mask (the plan view is the same for both masks) and FIGS. 1B and 1C respectively show sections, each taken along a chain line A-A in FIG. 1A, in terms of two examples.
In FIGS. 1A to 1C, each auxiliary pattern type phase shift mask comprises a transparent substrate 101 and a light-shielding layer 102 formed thereon, wherein the light-shielding layer 102 is formed with a main opening portion (contact hole) 103 and a plurality of auxiliary opening portions 104 located at peripheral portions of the main opening portion 103. It is configured such that light having passed through the main opening portion 103 and light having passed through each auxiliary opening portion 104 have a phase difference of approximately 180 degrees. For this purpose, in the example shown in FIG. 1B, the transparent substrate 101 has an etched portion 105 etched to a predetermined depth in a region corresponding to the main opening portion 103. On the other hand, in the example shown in FIG. 1C, the transparent substrate 101 has etched portions 105, each etched to a predetermined depth, in regions corresponding to the auxiliary opening portions 104, respectively. The auxiliary opening portions 104 are formed at predetermined positions and each have a fine line width so that the light having passed through each auxiliary opening portion 104 does not resolve a resist on a substrate to which a pattern is transferred.
FIGS. 2A to 2G are process diagrams showing a conventional phase shift mask manufacturing method.
For manufacturing an auxiliary pattern type phase shift mask like that shown in FIG. 1C, a light-shielding layer 102 and a first resist film 106 are first formed on a transparent substrate 101 in the order named as shown in FIG. 2A. Then, as shown in FIG. 2B, the first resist film 106 is written with a pattern corresponding to a main opening portion 103 and a plurality of auxiliary opening portions 104 by the use of, for example, an electron-beam writing apparatus and then developed, thereby forming a first resist pattern 107. Then, the light-shielding layer 102 is etched using the first resist pattern 107 as a mask, thereby forming a light-shielding layer pattern 108 having the main opening portion 103 and the auxiliary opening portions 104. Thereafter, as shown in FIG. 2C, the remaining first resist pattern 107 is stripped.
Then, as shown in FIG. 2D, a second resist film 109 is formed on the light-shielding layer pattern 108. Subsequently, as shown in FIG. 2E, the second resist film 109 is written with a pattern corresponding to the auxiliary opening portions 104 by the use of, for example, the electron-beam writing apparatus and then developed, thereby forming a second resist pattern 110. Then, the transparent substrate 101 is etched using the second resist pattern 110 as a mask, thereby forming etched portions 105 as shown in FIG. 2F. Thereafter, as shown in FIG. 2G, the remaining second resist pattern 110 is stripped, thereby completing an auxiliary pattern type phase shift mask.
In the manufacturing method shown in FIGS. 2A to 2G, the transparent substrate 101 is etched in regions corresponding to the auxiliary opening portions 104. However, the same manufacturing method is also applied to the case where the transparent substrate 101 is etched in a region corresponding to the main opening portion 103. That is, in FIG. 2E, the second resist film 109 is written with a pattern corresponding to the main opening portion 103 and then developed, thereby forming a second resist pattern. Subsequently, the transparent substrate 101 is etched using this second resist pattern as a mask, thereby forming an etched portion 105 like that shown in FIG. 1B.
Following the miniaturization and higher integration of LSIs in recent years, pattern data amounts for manufacturing phase shift masks have been increasing. Particularly, in each of the auxiliary pattern type phase shift masks as described above, since four auxiliary opening portions are required with respect to one main opening portion (contact hole) as shown in FIG. 1A, the pattern data amount increases four times.
When the pattern data amount increases, a long time is required for data processing and, further, a long time is also required for writing on a resist film. Particularly, in recent years, since the electron-beam writing method has shifted from the raster scan type to the vector scan type for improving the pattern accuracy, an increase in pattern data amount directly leads to an increase in the number of figures to be written, thus directly leading to an increase in writing time. Accordingly, it is desired to reduce the pattern data amount as much as possible, thereby minimizing the writing time.
Further, since an auxiliary opening pattern that is not resolved on a substrate to which a pattern is transferred is arranged around a main opening portion (contact hole), there arises a problem in mask fabrication due to limitation to a layout manner or occurrence of an irregular layout depending on design of an LSI.
For example, as shown in FIG. 3A, there is a case where a pattern layout is required in which a plurality of combinations each having a main opening portion 103 and auxiliary opening portions 104 are arranged so that some of the auxiliary opening portions 104 are adjacent to each other. That is, the main opening portions 103 are respectively arranged at portions corresponding to four corners of a diamond shape and four auxiliary opening portions 104 are arranged around each main opening portion 103. Therefore, some of the auxiliary opening portions 104 must be arranged adjacent to each other. In this case, as shown in FIG. 4A, a positive-type resist film 109 is formed on a light-shielding layer pattern 108, then, as shown in FIGS. 3B and 4B, the positive-type resist film 109 is written with a pattern corresponding to the auxiliary opening portions 104 by the use of an electron-beam writing apparatus and then developed, thereby forming a resist pattern 110. In this case, a region surrounded on all sides by openings corresponding to the auxiliary opening portions 104 is generated in the resist pattern 110. Accordingly, the resist pattern 110 has an extremely thin portion and, therefore, as shown in FIGS. 4C and 4D, there is a possibility that this portion falls down during development to cover, for example, one of the auxiliary opening portions 104. If, in this manner, the part of the resist pattern 110 falls down to close the opening portion, a transparent substrate 101 is not etched at the closed portion as shown in FIGS. 4D and 4E, resulting in occurrence of a defect.